The operating lifetime of a vehicle tire depends to a great extent on maintaining a proper pressure of air within the tire. For example, underinflation causes an increase in the flexing in the tread area, producing heat through friction and a resulting higher temperature of the tire which increases the rate of tire deterioration. In addition to the economic aspects of decreased tire lifetime, there are the safety aspects of operating a vehicle with underinflated tires. Overinflation, while it may result in a lower operating temperature, causes non-uniform tire wear and thus decreases the useful lifetime of a tire.
Although the advantages of maintaining tires properly inflated are well-known, there is a tendency on the part of many individuals to neglect tire maintenance. At least part of this attitude may result from the fact that modern tires require little attention and can remain at or near a proper inflation pressure for extended periods of time. A system for indicating tire pressure which is incorporated into the tire itself would make it more convenient for vehicle operators to monitor tire pressures, and such systems have been proposed in the prior art. However, some of these systems require substantial modifications to the tire, to the wheel or even to the vehicle itself. Others, such as those having an entire system which is mounted on the valve stem, can be difficult to read with accuracy.
U.S. Pat. No. 3,592,218 to Guy et al discloses a valve stem-mounted device which is described as combining the functions of a tire gauge and tire inflator. The tire gauge includes a slidable spring-biased piston which is responsive to air pressure when the gauge is activated. This device has no electrical components, and has limitations inherent in purely mechanical instruments. For example, instrumentation based only on mechanical properties are less flexible in the features which can be incorporated in the gauge.
U.S. Pat. No. 4,119,944 to Smith discloses a system for measuring air pressure in the tire of a moving vehicle and indicating that pressure to the vehicle driver. The system includes a wheel-mounted transducer, a chassis-mounted sensor and a chassis mounted electronic processing unit for converting sensor signals to tire pressure indications.
U.S. Pat. No. 4,137,520 to Deveau discloses a system for indicating tire pressure in which a tire pressure indicator is secured and disposed in the interior of the tire and mounted on the portion of the tire valve stem which is within the tire. The tire pressure indicator includes a pressure-tight bellows container which is preloaded to a selected gas pressure and which contains a strain gauge, a telemetering signal transmitter and a battery power source.
U.S. Pat. No. 4,494,106 to Smith et al discloses a system for continuously measuring tire pressure and which includes signal generating means mounted on the inside of the rim of a vehicle wheel.
U.S. Pat. No. 4,529,961 to Nishimura discloses a wheel-mounted tire pressure sensor which is mounted in the hub cap of a vehicle wheel. Pressure in the tire activates a sensor which is mounted within the hub cap of the tire to be monitored. The apparatus includes means for generating and broadcasting an electromagnetic signal which is representative of tire pressure, and means for receiving the signal by the vehicle operator.
U.S. Pat. No. 4,581,925 to Crutcher discloses a tire pressure indicating system using valve stem-mounted transducers connected to a hub-mounted display device.
An improved system for measuring and indicating tire pressure which can readily be added to a conventional tire without modifying the tire, the wheel or the vehicle and which provides a readily legible display of the pressure within the tire is desirable.